Difference between a no-arg constructor and a default constructor in Java

问题

Actually I can not understand that what is the difference between a no-arg constructor and a default constructor.

import javax.swing.*;

public class Test extends JFrame {
   public Test() {
     super();
     this.setSize(200,200);
     this.setVisible(true);
   }
   public static void main(Sting[] arg) {
       Test cFrame = new Test();
   }
}

Does this invoke the default constructor of this class while creating Test object called cFrame?

回答1:

The default constructor is a no-args constructor that the Java compiler inserts on your behalf; it contains a default call to super(); (not supper()) which is the default behavior. If you implement any constructor then you no longer receive a default constructor.

JLS-8.8.9. Default Constructor says (in part),

If a class contains no constructor declarations, then a default constructor with no formal parameters and no throws clause is implicitly declared.

If the class being declared is the primordial class Object, then the default constructor has an empty body. Otherwise, the default constructor simply invokes the superclass constructor with no arguments.

回答2:

The default constructor is a constructor that the Java compiler adds to your code if no explicit constructor is available. The default constructor invokes the super class constructor with no args.

If you have added your own constructor (no matter whether it's without parameters or with parameters) the compiler will not add the default constructor in this case.

回答3:

What is a default constructor ?

It is a constructor that is added by the compiler if you have not defined a constructor.

If your class has a constructor already then the compiler will not add the default constructor.

So in your case you have the constructor,

public Test(){
     super();
     this.setSize(200,200);
     this.setVisible(true);
   }

So there is no default constructor now to be invoked by the JVM.

回答4:

Answer is No. Reference variable cFrame will call non-arg constructor Test(), not default constructor. While Test() constructor will further find and call non-arg constructor of JFrame class(Parent) and so on Every class must have at least one constructor. If not declared explicitly, java compiler provides a non-parameterised constructor, i.e, default constructor. This default constructor calls its parent class’s non-parameterised constructor It initializes class variables to their default values.

Eg:

Class Base {}

Class Derived extends Base {} // Default constructor of class "Derived" will automatically calls non-arg constructor of class "Base" and intialzes value to the variables

While non-arg constructor is defined by a programmer only. It can also intializes the variables. One more point to add here is that super() call is automatically added by the java compiler, if doesn’t find super() in derived class.

Eg:

Class Base {

int y;
    public Base() {
    }
    public int getY() {
    return y;
    }

}

public class Derived extends Base {
 private int x;

 public Derived() { //super() will be automatically used
 }

 public int getX() {
 return x;
 }

 public void setX(int x) {
 this.x = x;
 }
}

Derived d = new Derived();
 System.out.println("Base x value => " + d.getX());

System.out.println("Base y value => " + d.getY());

Result:

Base x value => 0 // Default value to its primitive datatype(In this case: int)

Base y value => 0

回答5:

Answer to your question is No. Java won't provide a default constructor if you write any kind of constructor in class.

One difference between them is that the body of default constructor will always be empty whereas we can insert our own code in no-arg constructor.

来源：https://stackoverflow.com/questions/27654167/difference-between-a-no-arg-constructor-and-a-default-constructor-in-java

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java

constructor

default-constructor